The goal of the proposed research is to evaluate neuroendocrine mechanisms that coordinate seasonal reproduction in deer mice (Peromyscus maniculatus). Individuals of this long-day breeding species display reproductive involution in short days, followed by photorefractoriness and spontaneous gonadal recrudescence, the onset of which is regulated by an endogenous interval timer (IT). Whereas many studies have addressed factors mediating autumnal regression, little is known about neuroendocrine changes that permit accurate timing of photorefractoriness. These studies assess the functional significance of biological clocks by testing whether latitudinal differences in the onset of vernal breeding are associated with differences in the period of the IT in wild rodents. Deer mice will be housed in winter photoperiods to trigger reproductive regression. The timing of refractoriness will be compared between groups of mice derived from different northern latitudes. Follow-up experiments will specify the relevant photoperiodic experiences that determine the duration of the IT, whether the IT has a heritable genetic basis, and the response of the IT to artificial selection. A related set of experiments will address the physiological bases of photorefractoriness. Immunocytochemical techniques will be used to quantify the number, size, and density of GnRH-immunoreactive neurons after photorefractoriness has been triggered in short days, and patterns of GnRH mRNA expression will be compared between photorefractory and photoresponsive individuals via in situ hybridization. Experiments will determine whether changes in subpopulations of hypothalamic GnRH neurons mediate photorefractoriness, or alternatively whether photorefractoriness is a sequence of changes in pituitary responsiveness to GnRH. These studies will combine ecological and molecular techniques to assess neuroendocrine mechanisms governing reproductive variation and seasonal timekeeping.